The present invention relates to a video telephone system and a terminal device for the system which encodes a video image to communicate the encoded video image, and in particular, to a video telephone system and a terminal device for the system which conducts communications between terminal devices different in video processing capacity from each other.
The standards for video telephone systems include, for example, H.323 recommended by the telecommunication standardization sector of international telecommunication union (ITU-T). In association with the recommendation, the following recommendations of ITU-T are used to construct a video telephone system.
The recommendations are H.225.0 for call establishment, resolution of a communication party address, control-for communicability, and control for a band pass; H.245 to primarily negotiate terminal device's capability such like an encoding method of video and audio signals communicated between terminal devices, H.261 and H263 to encode video signals for transmission, and G711, G722, G.723.1, G728, and G729 to encode audio signals for transmission.
It is determined by H323 recommendation to use a realtime transport protocol (RTP), an RTP control protocol (RTCP) and TCP/UDP/IP as a transmission protocol of encoded audio/video data and a flow control protocol for the audio/video data. In conformity with the recommendations and protocols, the video telephone system can be constructed.
Particularly, H.245 recommendation allows a terminal device on a receiving side to notify, to a terminal on a sending side, an encoding method of video and audio signals which can be decoded by a terminal device on a receiving side. Moreover, the receiving side terminal device can specify as a decodable video size thereof, for example, a size determined for a common intermediate format (CIF) or a quarter CIF (QCIF) to notify the size to the sending side terminal device. Therefore, video signals can be communicated between terminal devices of mutually different types.
However, the display screen size varies between terminal devices and is indicated in units of pixels, namely, by various numbers of pixels. For example, some portable telephones and portable terminal devices of personal digital assistants (PDA) have a small-sized display screen in consideration of higher portability and reduction of production cost thereof. Conversely, the display screen is enlarged for higher visibility in some cases. Appropriate resolution of pictures also varies in units of pixels. When video signals of the size of CIF or QCIF are encoded to be transmitted to such a terminal device, performance of the terminal device cannot be fully obtained. Or, the sending side terminal device encodes a large amount of video signals and the receiving side terminal device decodes the signals including unnecessary signals. That is, the terminal devices must execute unnecessary processing. According to H.245, the display screen size of the receiving side terminal device can be notified as an optional parameter to the sending side terminal device. However, H.245 does not imply any use of the size notification to reduce the amount of video processing or displaying.
In some terminal devices, performance of the memory and the processor mounted on the device is restricted to reduce the price thereof. Resultantly, the decodable video size as well as the decodable video frame rate, the decodable bit rate, and the key frame insertion interval are restricted depending on cases. In this situation, when video signals received by a terminal device exceed the limitation of processing performance of the terminal device, an image produced using the video signals moves in an unnatural way. In a worst case, the received video signals cannot be decoded to display the image thereof.
In the description, “frame rate” is the number of pictures per second and “bit rate” is the amount of data per second expressed in units of bits. Moreover, “key frame” is a picture which is selected from the pictures of a video image for prediction and which can be decoded without referring to other pictures.
For video processing in relation to communication of video signals by terminal devices, when a sending side terminal device sends video signals, the terminal device must enlarge/minimize a video image obtained according to a size decodable by a receiving side terminal device to encode the video image. In addition, the sending side terminal device must enlarge/minimize a video image transmitting to the receiving side terminal device, according to a size of the own video display area of a display screen of the sending side terminal device to display the video image in the own video display area. At the same time, it is required that the received video signals are decoded and are enlarged/minimized according to a size of a video display area of the communicating party to display the video image in the communicating party video display area. This requires a large amount of memory areas and high processing performance. Consequently, there arises a problem that the terminal device is expensive.
There exist proposals for solving problems in the communication of video signals between terminal devices.
For example, JP-A-2001-069472 notifies a display mode on a receiving side to a sending side. However, in the article, a change of a display mode is notified to the sending side on the basis of a request from a user on the receiving side, and video processing performance of a terminal device on the receiving side is not notified to a terminal device on the sending side.
For example, according to JP-A-11-215481, a terminal device on a receiving side specifies a frame rate for a terminal device on a sending side. However, the frame rate is one of the attributes of the video processing method. According to the article, video processing performance of the receiving side terminal device is not notified to the sending side terminal device. Moreover, information regarding a display screen size of the receiving side terminal device is not notified to the sending side terminal device.
For example, JP-A-11-112955 discloses that information of a display screen size of a terminal device on a receiving side is notified to a terminal device on a sending side. However, the article copes with an image window in which a screen size of a communicating terminal device continuously changes with a lapse of time. An object of the technique is improvement in efficiency of communication lines, and video processing performance of the receiving side terminal device is not notified to the communicating terminal device.
Apart from the problems described above, it is necessary to appropriately recognize processing performance of the own communicating terminal device in the communication of video data therebetween.